#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <math.h>

int main()
{
	double x1, y1;
	double x2, y2;
	scanf("%lf%lf%lf%lf", &x1, &y1, &x2, &y2);
	printf("%.3lf", sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2)));
	return 0;
}